Method and apparatus for erasing developed images



Oct; 29, 1963 s. HOWELL METHOD ANQ APPARATUS FOR ERASING DEVELOPEDIMAGES 2 Sheets-Sheet 1 Filed Dec. 24, 1959' fine -B AT PAOLI INVENTOR.

RICHARD S. HOWELL flw WW AGENT Oct. 29, 1963 R. s. HOWELL 3,108,395

METHOD AND APPARATUS FOR ERASING DEVELOPED IMAGES Filed Dec. 24, 1959 2Sheets-Sheet 2 INVENTOR.

RICHARD s. HOWELL AGENT United States Patent 3,108,895 METHOD ANDAIPARATUS FUR ERASING DEVELOPED IMAGES Richard S. Howell, King ofPrussia, Pa, assignor to Burroughs Corporation, Detroit, Mich acorporation of Michigan Filed Dec. 24, 1959, Ser. No. 861,927 Ciaims.(Cl. 117l9) This invention relates to electrostatic reproduction andapparatus therefor.

The art of electrostatic printing consists basically of the productionon a charge-retentive surface of a pattern of electrical charges Whosedistribution is later indicated by the adherence to the charged areas ofa visible substance analogously denoted as ink, although it is commonlya particulate or powdered non-liquid. The application of the ink to thecharge-retentive medium in such fashion that it adheres visiblydifferently to the charged areas than to the non-charged or oppositelycharged areas is known, by obvious analogy, as development; and anypno-wss aimed at making the adherence of the ink permanent even afterdissipation of the charges which originally caused its adherence isknown, by similar analogy, as fixing.

Some references pertaining to the art of electrostatic printing are thefollowing applications for patents; they have all been assigned to theassignee of the present invention: United States patent applicationSerial No. 443,646, filed July 15, 1954, by Epstein and Innes, and nowPatent No. 3,012,839; United States patent application Serial No.714,767, filed February 12, 1958, by Epstein and Benn. A publication isThe Electro'graphic Recording Technique, by H. Epstein, Proceedings ofthe Western Joint Computer Conference, March 1955.

Most of the efforts of the practitioners and investigators of the art ofelectrostatic printing have been aimed at the creation of chargepatterns and the attachment of ink. However, some transfer processes areknown in which p articulate ink formed into a pattern on a dielectricsurface by the electrostatic process is then transferred by somemechanical means to another base, and the residual charge on thedielectric must be removed to prepare the dielectric for reuse by theformation of a different pattern of charges upon its surface. Theremoval of charge is often performed by ionizing the air adjacent to thedielectric surface, either by corona discharge or by radioactiveemanations; or the dielectric may be brought into an extremely humidatmosphere, or even immersed in a conductive liquid.

The simultaneous removal of ink image and charge pattern is desirablewhere it is purposed to examine the developed image only briefly andthen destroy it and its associated charge pattern and reuse therecording medium. Such mode of operation is taught in a copendingapplication for United States patent by Percy J. Barnes, entitledElectrostatic Data Display, Serial No. 861,906, filed December 24, 1959,and asigned to the assignee of this application. The excellentinsulating qualities of many modern dielectrics, such, for example, asthe plastic sheet material known in the trade as Mylar, render thedestruction of charge patterns quite difficult. it has, for example,been found that a tape of such material preserved in an office atordinary humidities is capable of retaining a charge pattern for aquarter of a year and, upon development after that period, presenting aperfectly legible image. :My invention comprises a means and method forremoving particulate ink and stored charges simultaneously from a recordmedium.

It is described in application for United States patent Serial Number714,767, filed February 1%2, 1958, by Herman Epstein and Robert E. Benn,Electrographic Recording Process, which application is assigned to theasignee of this application, how a pattern of charges deposited upon adielectric medium having a conductive backing may be rendered visible,or developed, by being drawn through a mass of electrically conductiveparticulate ink which mass is in electrical connection with theconductive backing of the medium. It is an observed fact, essential tothe operativeness of the invention of the referenced patent application,that the electrically conductive particulate ink mass does not, as mighthave been expected a priori, merely discharge the dielectric medium andleave it to pass from the mass of ink devoid both of charges and visiblepatter-n; instead, ink particles cling to the dielectric surfacewherever charges have been deposited on the dielectric, thus renderingvisible the charge pattern. This effect is not a critical one,producibl-e only sporadically and with painstaking control of criticalconditions, but a highly reliable and reproducible effect which has beenpreferred for use in reproducing equipment where reliability isessential.

My own invention is based upon the surprising discovery that adielectric medium bearing a charge pattern rendered visible bydevelopment with particulate ink, by the process described in thepreceding paragraph of this specification, may be divested both of itsvisible pattern of particulate ink and of its invisible pattern ofelectric charges by passing it a second time through a mass ofconductive ink identical with that employed in the original developingprocess.

Thus it is an object of my invention to erase both the visible ink imageand the invisible electric charge pattern by simple mechanical means notrequiring the use of radioactive material or high voltages.

Other objects and advantages of my invention will become apparent tothose skilled in the art in the course of the following detaileddescription and specification of the preferred mode of practicing myinvention.

For the better explanation of my invention 1 provide figures of drawingas follows:

FIGURE 1 is a schematic view representing a recording station forapplying electric charge patterns to a dielectric tape surface, adeveloping station for rendering such patterns visible by adherence ofparticulate ink, a viewing station, and an erasing station for removingboth the ink and the charge pattern from the tape to render it availablefor reuse;

FIGURE 2 is a schematic view representing a second embodiment of theinvention including a recording station for applying electric chargepatterns to a dielectric tape surface, a developing station forrendering such patterns visible by adherence of particulate ink, aviewing station, and an erasing station located in the same ink chamberwith the developing station;

FIGURE 3 represents a dielectric medium bearing a charge pattern not yetdeveloped, and therefore represented by dotted lines;

FIGURE 4 represents a dielectric medium as in FIG- URE 3, but as itwould appear after development with particles of ink pigment adhering inthe region of the charge pattern; I

FIGURE 5 represcents a dielectric medium as in FIG- URE 3 and FIGURE 4,but after passage through an erasing station which leaves it divested ofboth charge pattern and ink particles; and

FIGURE 6 represents a front view, and FIGURE 7 represents a profileview, of a third embodiment of apparatus for practicing my invention.

In FIGURE 1, a dielectric record medium 10 in the form of a tape whichmay be of Mylar and may have an electrically conductive backing isrepresented as passing over drive drum 14, which is rotated clockwise bymeans not shown, and causes tape medium to pass to the right of thefigure between grounded electrode 13 and electrostatic recording head 12which, in consequence of the application. to its electrodes of voltagesfrom the apparatus represented by 11, deposits a pattern of invisibleelectric charges on the upper surface of the dielectric of record medium10. The charged medium passes around an idler 9 to developing station24, where a chamber 16 is represented as containing a mass of powderedparticulate electrically conductive ink 18 which becomes attached, byelectrostatic attraction, to the dielectric surface of medium 10 inthose areas where electric charges have been stored. Baflies 19,supported by means not represented, serve to prevent any accidentaladhesion of ink agglomerate from moving upward with the medium 10; andvibrator 20, represented as a rotating polygonal shaft, driven by meansnot represented, vibrates the medium 10 sufficiently to shake loose anyink accidentally adhereing to regions where no charge has beendeposited. The preceding brief summary of the operations ofelectrostatic printing and developing is descriptive of art previouslydisclosed, in much more detail, in the applications for United Statespatents by Epstein and Innes and Epstein and Benn, to which referencehas been made in the introductory portion of this specification.

An ink satisfactory for such uses as here described may be composed ofapproximately spherical particles having a range of diameters fromtwenty to eighty microns, having a specific gravity, referred to water,between three and four, and with each particle having at least its outersurface electrically conductive. A satisfactory form of such ink may beproduced by silvering, by the Brashear process, small glass sphereshaving the size range specified. An alternative product is approximatelyspherical particles of clay, of specified size range, coated and to someextent permeated with carbon by pyrolysis of hydrocarbon vapors; suchcarbon is conductive. Details of the production and use of such inks areto be found in application for United States patent Serial No. 784,300filed December 31, 1958, by David W. Seymour. Both the glass spheres ofthis above-described Brashear process silvered ink and the clayparticles of this above-described Seymour application ink have a meltingpoint above 2000 C. and as described above and in that Seymourapplication S.N. 784,300, the particles have a specific gravity notgreater than four.

FIGURE 3 represents a portion of the dielectric of ing station 24 altersthe appearance of the tape record medium 10 to that represented inFIGURE 4, where the charge patterns represented in FIGURE 3 appear asvisible patterns delineated by adherent ink particles which are retainedin place by the field of the electrical charges represented in FIGURE 3.It should be emphasized that the exact location and the'mechanism ofstorage of the electric charges in the dielectric of medium 10 are notknown at this time. It is known that passing the charged dielectricsurface of tape 10 over a grounded metal roller does not discharge theimage, since such treatment is standard in operative electric printerswhich are so designed that the latent charge pattern or image on thedielectric medium is developed after being passed over a groundedelectrically conductive roller. It is not known whether the charges arelocated on the surface of the dielectric of a medium such as 10 and arenot discharged by the metal roller because there is, in fact, onlynegligible actual contact between the dielectric and the roller; orwhether the charges are actually stored slightly below the surface ofthe dielectric and can flow only with difficulty and over an appreciabletime to the surface, to be discharged. At all events, it is anexperimental fact that simple forcible removal of the ink pigment from adeveloped dielectric record medium such as 10 removes or dissipates onlya negligible part of the electric charge pattern which originallyattracted the ink. Thus mere removal of the ink particle image does notplace the record medium 10 in condition for the deposition of a newcharge pattern, since the original charge pattern still persists.

The recording medium 10 with the developedvisible image thereon ispassed through a viewing station represented by a rectangular frame 26when a message as exemplified in FIGURE 4 may be shown. The tape may bemoved continuously at a rate of speed through the viewing stationsutlicient to understand the message or it may be intermittentlyadvanced so that the message bearing portion of the medium is stopped inthe viewing frame. After the developed medium 10 has been observedsufficiently in the viewing station 25, it moves to erasing station 25.At erasing station 25 a container 17 holds a mass of ink 18, into whichthe developed medium 10 is plunged. It passes over idler rollers 9 andbetween two sets of wipers 21, which may be of felt, and which strip anyink particles which may be adhereing to the medium. If the medium 10 isprovided with a conductive backing layer, the portion of the ink mass incontact with that'layer will constitute means to connect the back inglayer with the rest of the ink mass. An alternate path may be providedby the idlers 9 if they are of can ductive material and are inelectrical connection with the mass of ink 18 as, for example, throughcontainer 15. If the depth of the ink mass 18 is sulfic'iently greatthat its pressure near the bottom of container '17 prevents easy flow ofink particles through the mass, and thus shears the adherent inkparticles away from the moving tape 10,- the wipers may be dispensedwith. At any event, it is necessary that ink once brought into intimatecontact with the surface of medium 10 be forcibly removed, and that moreink 18 be brought in contact with the dielectric surface of medium 10and then forcibly removed. This process permits the tape medium 10 to beremoved from the mass of ink 18 at the end of its passage througherasing station 25 with no ink particles adherent to it, and devoid'ofits original charge pattern. It has been found that passage through fourinches of ink at a speed of about twenty inches per second is suflicientto' eliminate both'ink and charge pattern. It is believed that thisprocess works becausse the passage of the medium through the ink bringsink particles in contact with substantially every point of the surfaceof the medium 10, and that the stored charges in the medium are carriedaway by the ink particles which successively contact the surface.

The arrangement of the path of the medium 1% in FIG URE l is such that,after leaving the erasing station 25, the medium passes to drum 14 forfurther use. Thus, the recording medium may be an endless tape as shown,passing either intermittently or continuously, through the recordingstation represented by the head 12 and through the developing, viewingand erasing stations 24, 26 and 25 respectively. A surplus of ink 18will gradually be deposited in container 17 of the erasing station 25and may be returned by any suitable means to container 16 of thedeveloping station 24. This gradual contamination with developing ink ofthe conductive particles used for the erasing operation is the reasonWhy the ink 18 in both the developing station 24 and the erasing station25 has been represented by the same reference number; obviously it is tobe preferred in general that the same ink be: used in both so that thenominal contamination of the ink in the erasing station 25 is actuallynon-existent because the additions from the original content ofdeveloping station 24 are the same as the ink already present in erasingstation 25. It is possible to employ conductive particles devoid ofpigment for purposes of erasure, and, if they are of difierent densityfrom the particles of ink, the ink contamination may be removed from themass of erasing particles by dilferential flotation. The practicalusefulness of such a procedure would obviously be 801116 what limited,although it would effectively apply the principles of my invention.

An alternative arrangement of developing and erasing stations isrepresented in FIGURE 2. In FIGURE 2, the drive drum 14, groundelectrode 18, electrostatic recording head 12, apparatus 11 all performexactly the same functions as in FiGU RE 1 The record medium it? passes,after imposition of a charge pattern upon its dielectric surface,through a mass of ink 13 at developing station 22, the elements I? and2d of the developing station 24 and the viewing station 26 perform thesame functions as at those described in connection with FIGURE 1.However, erasing station 23 is embodied in the same receptacle orhousing 15 as is developing station 22. The medium 10, afterdevelopment, is carried up out of the developing station, passeshorizontally through station 26 for viewing, and then moves downwardagain to erasing station 23: where it plunges into the mass of ink id,moves horizontally between two wipers 21, which may be of felt, and thenturns upward again to move in av path which ultimately returns it todrum 14 for reuse.

An advantage of the embodiment of FIGURE 2 is that the ink removed fromthe medium at erasing station 23 will ultimately flow back to replenishthe supply at developing station 2-2, and thus the removed ink may bereused with no special provision to achieve this.

An extremely simple example of a combination developing and erasingstation is to be found in FIGURES 6 and 7, FIGURE 7 being a profile viewof the elevation of FIGURE 6, each being a section of the other.Actually, the eiiect of the sections is substantially as if a phantomdrawing had been made in which the nearer wall of container 34 wasremoved to permit free vision of the wor-ikng parts represented. Inthese figures, drum 69 is represented as supported on a shaft 7t) whichis supported by bearings and driven by rotating means not hererepresented. Record medium tape it? is passed three times helicallyaround drum 6?, and, during the clockwise (as represented in FIGURE 6)rotation of drum d9, the tape is carried for two turns past a wiper 211,passing through the mass of electrically conductive ink lid in theprocess of being erased. After its second trip past the wiper 2d, thetape medium 16 passes under the recording head 12, where a new messageis applied to it in the form of a pattern of invisible electric charges,which pattern is rendered visible by the adherence of ink i8 whichoccurs during the third trip of the medium it) through the mass of ink18, but without contact with the wiper The developed and thus visiblepattern is carried upwar by movement of the medium lit turns to movehorizontally for viewing at 26 and then moves downward again to the drum69 for repetition of the erase-record-develop cycle. Drum 69 may be ofconductive material, or have a surface of conductive material, whichwill make contact with any conductive backing layer of medium 1d andwill also make electrical contact with the mass of ink 18-. Asrepresented in FIGURES 1 and 2, the ink 1S ordinarily constitutes itsown means for making electrical connection with such conductive backinglayer. It is. quite possible to arrange the erasing station so that theback of the medium is not in contact with the mass of ink 33, requiringthe provision of more obvious connection means as here described.

It is obvious that a considerable variety of mechanical variations maybe made to practice this invention. Also, I have found that thisinvention is operative for dielectric media having an electricallyconductive backing, as taught in the aforesaid patent application byEpstein and Benn, but also functions to erase specific charge patternson dielectric media not having such a back ng. However, it is found thatrepeated erasures on media not having an electrically conductive backingtend to build up a background of electric charge which tends to producea somewhat dark background upon development. This does not preclude thepossibility of applying my invention to the erasure of patterns ofcharge upon such media, however, provided the number of repetitions oferasure in a given period is not too great. The advantages achieved bythe use of a conductive backing, as taught by Epstein and Benn, are sogreat that there is usually no great motive for employing a mediumwithout conductive backing. The disadvantages attendent upon erasure ofcharge patterns on a medium not having an electrically conductivebacking are comparable with those attendant upon its use as anelectrostatic recording medium, so that it may fairly be said that myinvention may be limited in its utility to the operativeness of themedium, not by any inherent limitations of the invention itself.

It is suspected that the operation of my invention is so effective as ithas in fact been proven to be because the passage of the dielectriccharge through the conductive ink particles permits electrical contactwith substantially all of the area of the dielectric surface. The inkparticles are approximately spherical in shape, and would thus not makedirect physical contact with more than a small part of the area whichthey obscure to view. Consequently, a continual rolling of ink particlesover the dielectric surface might be adequate to make conductiveelectrical contact with substantially all of the dielectric surface, andthus remove from the surface charges which cannot flow across thesurface to discharge conductors. However, this hypothesis must beevaluated in the light of the fact that simple rolling of the dielectricover a grounded metal roller does not destroy the stored charge pattern.

Having described the nature of my invention, and described in detailthree different embodiments of its practice, I claim:

1. In an electrostatic recording device comprising a dielectric recordmedium of high resistivity having an electrically conductive backinglayer, means to establish patterns of electrical charges on the exposedsurface of the said record medium, developing means comprising acontainer, :1 mass of electrically conductive particulate ink ofpredetermined amount contained in said container, and means to move saidrecord medium into and out of the said mass of electrically conductiveink With the said ink in intimate contact with the said exposed surfaceof the said record medium, and with the said electrically conductivebacking layer in electrical connection with the said mass of ink,whereby the said ink particles adhere to and render visible the saidpatterns of charges: means to move the developed medium back into andout of said developing ink and through a suflicient amount of saiddeveloping ink to effect substantially complete removal of both the saidvisible adhering ink and the said charges, and means to remove fromadherence to the surface of the said dielectric medium during itspassage back into and out of the said developing ink substantially allof the ink particles held adherently thereto by the said electricalcharges.

2. In an electrostatic recording device comprising a dielectric recordmedium of high resistivity having an electrically conductive backinglayer, means to establish patterns of electrical charges on the exposedsurface of the said dielectric medium, developing means comprising acontainer, a contained mass of electrically conductive particulate inkdisposed insaid container and means to move the said record medium afirst time into and out of the said mass of electrically conductive inkwith the said ink in intimate contact with the said exposed surface ofsaid record medium and with the said electrically conductive backinglayer in electrical connection with the said mass of ink, whereby thesaid ink particles adhere to and render visible the said pattern ofcharges: means to move the said record medium a second time into and outof the said mass of electrically conductive ink, with the said exposedsurface of the said record medium in intimate contact With the said\mass of electrically conductive ink,

means to make electrical connection during said second time between thesaid electrically conductive backing layer of said medium and thesaid-mass of electrically conductive ink, the amount of said ink andsaid container being in cooperating relationships such that said secondtime moving of said record medium is of sufficient duration to elfectsubstantially complete removal of said pattern of charges, and means torestrain particles of said ink from moving adherently with the movingsaid exposed surface of the said medium during said second time;

3. In an electrostatic printing system: a container and a mass ofelectrically conductive particles contained in said container; means formoving a dielectric record medium having an electrically conductivebacking layer and having a visible charge pattern of adherent inkparticles into and out of said mass of electrically conductive particlesin said container with the ink particles on the dielectric record mediumin intimate contact with said mass of electrically conductive particlesin the container to establish electrical connection between the saidelectrically conductive backing layer of said medium and the said massof electrically conductive. particles, the electrically conductiveparticles in the container being ink particles of low specific gravityand similar to said adherent visible ink particles; and stationaryWiping means operable during said intimate contact between the mass ofsaid electrically conductive particles and the said ink particles on thedielectric record medium for applying shearing force to restrain anyparticles from moving with the dielectric medium in its direction ofmotion, said mass of electrically conductive particles being of amountand said intimate contact and said motion restraining being of durationsufiicient and in coacting relationship to remove substantiaily all ofthe said charges of said charge pattern and said adherent ink particlesfrom said dielectric record medium by the time of its emergence fromsaid container.

4. In an electrostatic recording device having means for applying apattern of electric charges upon a chargeretentive dielectric surface ofa record medium composed of a substantially continuous film of asynthetic organic resin overlying an electrically conductive backing andfurther means for causing an electrically conductive particulate inkcomposed of electrically non-conductive particles having an electricallyconductive surface coating to adhere to the portions of said surfaceWhere the said pattern of electric charges exists: a chamber, a mass ofsimilar electrically conductive particulate ink contained in saidchamber, said last-named ink comprising coated substantially sphericalparticles of small diameter and of material of melting point andofspecific gravity not greater than four to enable erasing in saidchamber while enabling the adhering of the ink to said charge-retentivesurface without erasing in saidmeans for causing adherence of the ink tothe charge pattern portions of said surface, and means for bringing aportion of said similar electrically conductive mass of ink intointimate contact with said surface and for causing relativemotionbetween the charge-retentive surface and the portion of the massof ink while in electrical connection with the said electricallyconductive backing so as to expose said charge-retentive surface to theconductive ink surface of sufiicient ink particles to removesubstantially all of said charges as Well as said ink adhering to saidcharge-retentive surface.

5. A method of removing, from a record medium having an electricallyconductive backing and a coating of material having an el ctric chargeretentive surface hearing a record pattern of electric charges rendereddetectable by the adherence to the portions of said surface bearing thesaid pattern of charges of solid electrically conductive particles,substantially all of both the said pattern of electric charges and thesaid adherent electrically conductive particles, comprising: the step ofimmersing the medium in a mass of conductive particles of specificgravity of an order less than four by drawing the said charge-bearingrecord medium to pass it into and out of said mass of conductiveparticles and in intimate contact therewith to establish through theconductive particles and the backing an electrically conductive pathbetween the surfaces of the coating and the step of restraining the saidconductive particles from moving together with said record medium, saidrestraining step being interposed with providing said intimate contactfor at least a portion of the period of said immersing step, saidpassing of said record medium through said conductive particles inintimate contact being efiected a duration of time sufficient to causesubstantially complete removal of both said pattern of charges and saidadhering conductive particles.

References Cited in the file of this patent UNITED STATES PATENTS2,550,724 Sabel May 1, 1951 2,892,446 Olden June 30, 1959 2,911,330Clark Nov. 3, 1959 2,970,299 Epstein et al. Jan. 31, 1961 2,976,144 RoseMar. 21, 1961

5. A METHOD OF REMOVING, FROM A RECORD MEDIUM HAVING AN ELECTRICALLYCONDUCTIVE BACKING AND A COATING OF MATERIAL HAVING AN ELECTRICALCHARGE-RETENTIVE SURFACE BEARING A RECORD PATTERN OF ELECTRIC CHARGESRENDERED DETECTABLE BY THE ADHERENCE TO THE PORTIONS OF SAID SURFACEBEARING THE SAID PATTERN OF CHARGES OF SOLID ELECTRICALLY CONDUCTIVEPARTICLES, SUBSTANTIALLY ALL OF BOTH THE SAID PATTERN OF ELECTRICCHARGES AND THE SAID ADHERENT ELECTRICALLY CONDUCTIVE PARTICLES,COMPRISING: THE STEP OF IMMERSING THE MEDIUM IN A MASS OF CONDUCTIVEPARTICLES OF SPECIFIC GRAVITY OF AN ORDER LESS THAN FOUR BY DRAWING THESAID CHARGE-BEARING RECORD MEDIUM TO PASS IT INTO AND OUT OF SAID MASSOF CONDUCGIVE PARTICLES AND IN INTIMATE CONTACT THEREWITH TO ESTABLISHTHROUGH THE CONDUCITVE PARTICLES AND THE BACKING AN ELECTRICALLYCONDUCTIVE PATH BETWEEN THE SURFACES OF THE COATING AND THE STEP OFRESTRAINING THE SAID CONDUCTIVE PARTICLES FROM MOVING TOGETHER WITH SAIDRECORD MEDIUM, SAID RESTRAINING STEP BEING INTERPOSED WITH PROVIDINGSAID INTIMATE CONTACT FOR AT LEAST A PORTION OF THE PERIOD OF SAIDIMMERSING STEP, SAID PASSING OF SAID RECORD MEDIUM THROUGH SAIDCONDUCTIVE PARTICLES IN INTIMATE CONTACT BEING EFFECTED A DURATION OFTIME SUFFICIENT TO CAUSE SUBSTANTIALLY COMPLETE REMOVAL OF BOTH SAIDPATTERN OF CHARGES AND SAID ADHERING CONDUCTIVE PARTICLES.